cpu: TrafficGen as BaseCPU

cpu: TrafficGen as BaseCPU

TrafficGen has additional attributes to behave like a BaseCPU. Python
scripts that expect sim. objects derived from BaseCPU can now be used with
TrafficGen without additional modifications.

Change-Id: Iee848b2ba0ac1851c487b1003da9bd96253d291a
Signed-off-by: Tiago Muck <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18416
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>